The brushless DC motor is a mainstay of the data processing industry. This type of motor is widely used in low weight loading, high speed, precision control applications. One of the main usages for brushless DC motors is in hard disk drives in the data processing field.
The construction of brushless DC motors has long been known in the industry. These motors are constructed in many different phases and excitation modes. These differences are caused by modifications of the control circuitry, number of commutating devices and configurations of the rotor magnets, stator stacks and windings of the motor. A typical motor of one of these types is shown and described in the applicant's co-pending application for a "Magnetic Motor Tachometer Assembly", Ser. No. 06/429,076, filed Sept. 30, 1982. The motor shown therein is of the two phase multipolar excitation type.
The simplest type of brushless DC motor is the two phase unipolar excitation motor. In this type of motor current is passed through a given conducting wire in the winding in a single direction only. Two phase unipolar excitation motors are more easily and economically constructed than more complex versions. They are therefore a desirable type of motor for most applications.
One inherent problem with two/phase unipolar excitation brushless DC motors relates to the occasional inability of the motor to start automatically. Due to the magnetic stability factors discussed in detail below, it would frequently be the case that an unmodified brushless DC motor of this type would come to rest in a position from which it will not readily start upon application of induction current. Obviously, this is an undesirable situation.
One possible method of overcoming this difficulty is to apply mechanical or manual initial rotation to the rotor portion of the motor. Such an approach is not always feasible at all due to the inaccessibility of the elements controlled by the rotor and is occasionally even damaging to the equipment. Consequently, the method is not used.
Another approach which has been attempted in the prior art to overcome this difficulty is modifying the location of the magnetically stable rest points, known as detent positions, or zero torque nodes for the rotor. This approach is reflected in the motors marketed by the PAPST Company and the SINANO TOKKI Company. Each of these motors attempts to modify the magnetic detent positions by modifying the air gap between the rotor and the stator pole such that the rotor is urged towards one end of the pole.
The PAPST motor uses stator poles that are slanted with respect to the rotor such that there is a narrower air gap between the rotor and one end of the pole than the other end. In this manner, the magnetic attraction is stronger at the closer end and the rotor is urged towards the closer end of the pole. Since the rotor is then aligned off center on the pole the rest point is destabilized with respect to the application of electrical current and the motor will always start.
The SINANO TOKKI motor uses a similar principle in that each stator pole is divided into a longer portion and a shorter portion. The air gap between the rotor and the shorter portion of the pole is significantly greater than that between the rotor and the longer portion of the pole. Again, the permanent magnet on the rotor are urged towards the longer portion of the pole where the narrower air gap exists. This results in the same offset rest positions and instant starting as the PAPST motor.
The variable air gap methods of creating instant starting two phase unipolar excitation brushless DC motors has a significant disadvantage that the operation of the motor at normal high operating speed is degraded. Since a portion of the stator poles have a greater air gap than other portions there is an uneven magnetic field across the pole which tends to unbalance the rotor. Furthermore, the magnetic flux linked with the winding is limited by the increased air gap at a portion of the pole. This necessarily results in a reduction of potential motor output power.
No prior art method is presently known for providing an instant starting two phase unipolar excitation brushless DC motor which has optimum performance at normal operating speeds. The difficulties of instant starting, maximum power and good rotor balance have not heretofore been combined.